The radiation pattern of a handset (and generally of any handheld device that includes an antenna for receiving and transmitting electromagnetic wave signals) is determined, among other factors, by the antenna shape, its position on the handset, and also the handset size and its physical construction. Usually, the antenna is placed at an edge of the handset to maximize radiation. Such an edge is usually the top part of the handset (near the earphone) although can also be in some cases the bottom part (near the speaker). This way, the combination of such a position together with the size of the handset, and in particular the size of the grounding metals inside the handset (mainly printed circuit boards and electromagnetic shields), usually determine the shape of the pattern.
The example shown in FIG. 1 describes this fact. FIG. 1 is a simplified model of a handset, including a printed circuit board (PCB) for the conducting ground (1) (wider rectangle on the left), and a whip antenna (2) (narrow strip on the right) which is typically a quarter of a wavelength in length.
Typical radiation patterns for such a handset are shown in FIG. 2. Such a pattern shows a vertical cut (XZ plane) on the handset, with the top part (antenna) place on the right side of the horizontal X axis, and the handset body on the left size of same axis.
It is seen on such a plot that typically the radiation pattern is tilted towards the lower part of the handset. That is, radiation is enhanced below horizon (vertical axis on the graph) which is an inconvenient when receiving and transmitting from long distance base stations, since in these cases radiation comes from the vicinity of a horizontal plane (ZY plane). This phenomenon is related to the distribution of currents flowing on the handset, which are asymmetrically split between the antenna and the casing (PCB, shieldings) of the phone. Again, the antenna position, together with the PCB and handset size, are the determining effects contributing to this phenomenon.
This problem becomes even more relevant when the handset is of the clamshell or flip-phone type. In clamshells phones, the keyboard and screen are usually split in two parts that unfold apart by means of a hinge connecting said two parts. Both parts of the phone include metal parts (PCB, shieldings) and are interconnected by means of a flexible circuit or wire set. When such a type of handheld is unfolded, the overall length of the metal part (typically the PCB ground) is increased, which again influences the shape of the radiation pattern. This example of a handset and pattern distortion effect is shown in FIGS. 3, 4 and 5.
FIG. 4 shows a typical difference between the folded and unfolded phone radiation patterns in the horizontal plane (YZ). The unfolded phone radiates (and receives) a weaker signal (smaller circle) in the horizontal plane than the folded one. This is due to a pattern distortion in the vertical plane (XZ) as shown in FIG. 5. The new pattern displays a minimum radiation on the horizontal plane, while steering radiation to other for quadrants in space. This effect can be even more significant when a handset integrates a small internal antenna.
Some structures known in the prior art, such as multilevel structures, space-filling curves or the ground planes described in the PCT publication WO03023900, can be advantageously used in the present invention.
The PCT publication WO0122528 describes a multilevel structure for an antenna device consisting of a conducting structure including a set of polygons, all of said polygons featuring the same number of sides, wherein said polygons are electromagnetically coupled either by means of a capacitive coupling or ohmic contact, wherein the contact region between directly connected polygons is narrower than 50% of the perimeter of said polygons in at least 75% of said polygons defining said conducting multilevel structure. In this definition of multilevel structures, circles and ellipses are included as well, since they can be understood as polygons with a very large (ideally infinite) number of sides.
The PCT publication WO0154225 describes a space-filling curve SFC: as a curve composed by at least ten segments which are connected in such a way that each segment forms an angle with their neighbours, that is, no pair of adjacent segments define a larger straight segment, and wherein the curve can be optionally periodic along a fixed straight direction of space if, and only if, the period is defined by a non-periodic curve composed by at least ten connected segments and no pair of said adjacent and connected segments defines a straight longer segment. Also, whatever the design of such SFC is, it can never intersect with itself at any point except the initial and final point (that is, the whole curve can be arranged as a closed curve or loop, but none of the parts of the curve can become a closed loop). A space-filling curve can be fitted over a flat or curved surface, and due to the angles between segments, the physical length of the curve is always larger than that of any straight line that can be fitted in the same area (surface) as said space-filling curve. Additionally, to properly shape the gap according to the present invention, the segments of the SFC curves included in said multilevel structure must be shorter than a tenth of the free-space operating wavelength.
The PCT publication WO03023900 describes a ground-plane for an antenna device, comprising at least two conducting surfaces, said conducting surfaces being connected by at least a conducting strip, said strip being narrower than the width of any of said two conducting surfaces.